1. Field of the Invention
This invention generally relates to the light wave controlling technology and in particular to an optical switching device. More specifically, this invention relates to optical switching devices which carry out switching operation utilizing the temperature dependent characteristic of the index of refraction of crystalline materials.
2. Description of the Prior Art
In the field of optical information processing such as an optical communication system including optical memory devices and optical printers, devices for switching the transmission of light on and off are required. Such optical switching devices are desired to have minimum cross-talks, switching capability at lower power, high speed switching performance, low manufacturing cost, etc. Some of the prior art optical switching devices have been produced by utilizing the acoustooptic effect and others have been produced by utilizing the electrooptic effect.
FIG. 1 illustrates a prior art optical switching device 1 utilizing the electrooptic effect. FIG. 2 is a cross-sectional view of the optical switching device taken along I--I line in FIG. 1. As shown, the device 1 includes a substrate 6 made of a dielectric crystalline material such as LiNb0.sub.3 and a planar waveguide 7 in the shape of "Y" having a higher index of refraction than that of the substrate 6 and formed in the surface of the substrate, for example, by Ti diffusion. The waveguide 7 includes an inlet waveguide section 7a and a pair of outlet waveguide sections 7b and 7c which branch out from the inlet section 7a. Also provided is an insulating layer 9 of SiO.sub.2 or the like formed on the surface of the substrate covering the branching out section of the waveguide 7. On the insulating layer 9 are formed control electrodes 8a, 8b, 8c and 8d.
The control electrode 8b is located immediately above and in commensurate in shape with the waveguide sections 7a and 7b; while, the control electrode 8c is located immediately above and in commensurate in shape with the waveguide sections 7a and 7c. On the other hand, the control electrode 8a is formed similar in shape to and located in side-by-side relation with the electrode 8b, and its location is not immediately above the waveguide 7. The control electrode 8d is similarly arranged.
Under the condition, if a d.c. voltage is applied between the electrodes 8b, 8d and the electrodes 8a, 8c, an electric field directed in the direction of C axis of the dielectric crystal 6 is formed in the waveguide sections 7a, 7b and 7c, as best shown in FIG. 2. As a result, a difference in the indexes of refraction is produced between the waveguide sections 7b and 7c so that the light waves introduced into the waveguide section 7a come to propagate mostly into either one of the branching out sections 7b and 7c. That is, the light waves will propagate into the outlet waveguide section having a higher index of refraction. Thus the direction of propagation of light waves may be controlled by controlling the direction of voltage to be applied to the electrodes.
However, in the above-described optical switching device, sophisticated techniques are required for fabrication of the control electrodes since they must conform in shape to the waveguide. Furthermore, if the waveguide is to be structured for the single mode application, it is necessary to form a waveguide having a width in the order of 5 microns and thus difficulty will be encountered in the formation of control electrodes. Moreover, since the spacing between the adjacent electrodes will be extremely small in such application in which a d.c. voltage of some 10 volts will be applied to the control electrodes, a problem in voltage resistant characteristics will loom. It is also to be noted that a device design is rather limited because a d.c. voltage source is required as a source for driving the device.
Another prior art optical switching device 2 is illustrated in FIG. 3. This device includes a substrate 6 of a crystalline material such as LiNbO.sub.3 and a slab 7 of Ti-diffused layer formed at the surface of the substrate 6 as a waveguide. A prism 10 of TiO.sub.2 is mounted at one end of the slab waveguide 7, at the other end of which is provided an insulating layer 9 comprised of a material such as SiO.sub.2, Al.sub.2 O.sub.3, etc. On the insulating layer 9 are provided three parallel planar electrodes 8e-8g with their longitudinal directions in parallel with the longitudinal direction of the substrate 6. These electrodes 8e-8g are electrically connected to terminals 11a-11c, respectively, and these terminals are connected such that an appropriate d.c. voltage may be applied between the terminal 11b and the commonly connected terminals 11a and 11c.
With such a structure, when light waves are introduced into the slab waveguide 7 through the prism 10, they propagate toward the position where the electrodes 8e-8g are formed. Then, upon formation of an electric field to increase the index of refraction of that portion of the slab 7 immediately below the central electrode 8f by applying an appropriate voltage between the central electrode 8f and the side electrodes 8e and 8g, the light waves introduced are collected at and emitted from the location immediately below the central electrode 8f. On the contrary, if the voltage applied between the central electrode 8f and the commonly connected side electrodes 8e and 8g is reversed in polarity, the incident light waves will be distributed across the slab waveguide 7 and thus the intensity of the light emission from that portion of the waveguide in the vicinity of the central electrode 8f is significantly decreased. In this manner, depending upon the polarity and magnitude of a voltage applied between the central electrode 8f and the side electrodes 8e and 8g, the intensity of light emission from a particular location of the waveguide 7 varies. Accordingly, the device 2 of FIG. 3 can carry out optical switching.
However, in the above-described prior art device of FIG. 3, since the waveguide 7 is in the form of a slab, the direction of propagation varies depending upon the incident position and direction of incident light waves to the waveguide 7 so that switching characteristics are rather unstable and difficulty exists in fabricating an optical switching device of excellent quality. Moreover, provision of the insulating layer 9 is called for to limit the propagation loss of the light wave against the electrodes 8e-8g. Besides, the wide width of the slab waveguide 7 precludes the possibility of application to the single mode operation. In addition, narrow spacing between the adjacent electrodes makes it difficult to manufacture, and the required use of a d.c. voltage as a source also presents some disadvantages as discussed before.
As mentioned previously, it has also been proposed to provide an optical switching device with the utilization of acoustooptic effect. However, such a device is disadvantageous in that it tends to be expensive because it requires a crystalline material such as PbMoO.sub.4 and TeO.sub.2 and a transducer for converting an electrical signal into a ultrasonic signal. This prior art device is also disadvantaged in that it requires a significantly large driving power at high frequencies. Thus it is economically disadvantageous as well as limited in usage.